1. Field of the Invention
The present invention relates to an image processing apparatus, a method for processing an image, and a storage medium therefor.
2. Description of the Related Art
The electrophotographic method is known as an image recording method used for image processing apparatuses such as printers and copying machines. The electrophotographic method forms a latent image on a photosensitive drum by using a laser beam and develops the latent image by a charged color material (hereinafter referred to as toner) to form a toner image. To record an image, the electrophotographic method transfers the developed toner image onto a transfer sheet and then fixes the transferred image.
Conventionally, when an image forming unit employs the electrophotographic method, electrophotographic processes such as laser exposure, latent image formation on a photosensitive member (drum), image development by toner, toner image transfer onto paper, and image fixing by heat are susceptible to the apparatus ambient temperature and humidity as well as to aging of apparatus components. Therefore, the toner amount to be finally fixed onto paper changes each time. Such instability is not specific to the electrophotographic method but known to similarly occur in the ink-jet method, heat transfer method, and other various recording methods.
To solve such issue, a certain technique outputs a test pattern image from an image processing apparatus, measures the density of the output test pattern, and corrects the characteristics of the image forming unit based on the measured density. With a method for correcting nonlinear characteristics used in this technique, a look-up table is generally used.
With this method, a maximum density targeted by the apparatus (hereinafter referred to as target maximum density of the apparatus) is set in advance and the nonlinear characteristics are corrected aiming to smoothly achieve intended gradation up to the maximum density. However, since the maximum density may be exceeded depending on the apparatus state, the excessive density may be restrained by using the above-described look-up table. In this case, restraining an upper-limit density may cause a problem that jaggies or discontinuities of images are produced when outputting texts and thin lines having the maximum density. To solve this problem, a technique has been discussed which enables a user to select a look-up table for forcibly correcting an upper-limit density to a maximum value to ensure that an input with a maximum density be output with the maximum density (for example, Japanese Patent Application Laid-Open No. 2001-094784).
Meanwhile, if the amount of recording material such as toner and ink exceeds a certain fixed amount per unit area, inferior fixing or scattering of toner may occur possibly causing degradation in image quality as well as damage to the apparatus. Therefore, the total recording material amount per unit area is controlled so as to constantly fall below a preset fixed amount. In the case of a color printer, for example, to prevent excessive toner amount reduction, a certain method reduces the total toner amount per unit area by changing the combination of four colors (cyan, magenta, yellow, and black) of toner. More specifically, it is common to increase the amount of black toner, instead of reducing an equal amount of cyan, magenta, and yellow toner, to minimize change in tint and degradation in gradation.
However, as described above, if an output exceeds the target maximum density of the apparatus to reduce jaggies under the situation that the upper-limit recording material amount per unit area is predetermined, image defects such as inferior fixing and scattering of recording material may occur.
For example, suppose that, when the target maximum density of the apparatus is estimated to be 1.5 and the apparatus outputs an image having a density of 1.6 in a certain environment. This case will cause excessive recording material. With a technique discussed in Japanese Patent Application Laid-Open No. 2001-094784, although jaggies are reduced, the recording material amount per unit area may exceed the upper limit if a maximum density output is permitted. (Hereinafter, toner, ink, and other recording materials will be collectively referred to as toner.)
If a toner amount limit is set in a small value by estimating much increase in density of the image, when an image to be printed has a low density, a toner amount may be excessively reduced depending on the printer characteristics. Increasing the toner amount generally increases the image density and color depth, enabling more desirable color reproduction. Therefore, it is said that minimizing the toner reduction amount produces a preferable image quality.